Media captionAnna Holligan is the only foreign journalist travelling on board the Norwegian frigate

Norwegian and Danish ships waiting to remove Syria's chemical weapons are returning to port in Cyprus, signalling a key deadline will not be met.

Bad weather, shifting battle lines and road closures are being blamed for the delay.

The international mission is waiting for Syria's most dangerous chemicals to be transported to the port in Latakia.

The deadline is the first milestone of a deal to rid Syria of its chemical weapons arsenal by the middle of 2014.

It was brokered by the US and Russia after rockets filled with the nerve agent sarin were fired at three towns in the Ghouta agricultural belt around Damascus on 21 August, killing hundreds of people.

Western powers said only Syrian government forces could have carried out the attack, but President Bashar al-Assad blamed rebel fighters.

'On high alert'

Under the international disarmament plan, US satellites and Chinese surveillance cameras are to track the progress of Russian armoured lorries as they carry the chemical weapons from 12 storage sites in Syria to Latakia, on Syria's Mediterranean coast.

Danish and Norwegian cargo ships will then transport the chemicals to a port in Italy, where they will be loaded on to the US Maritime Administration vessel MV Cape Ray and taken out into international waters before being destroyed by hydrolysis.

However, the BBC's Anna Holligan, who is travelling on board a Norwegian frigate HNoMS Helge Ingstad, reports that the European ships are docked in Limassol, Cyprus on the day they are supposed to be escorting Syria's most dangerous chemicals out of the country.

The vessels left Limassol on Saturday but turned back on Tuesday after the hazardous containers failed to arrive for collection in Latakia. Now the plan is to refuel in Limassol before returning to sea in the coming days.

"We are still on high alert to go into Syria," Norwegian defence ministry spokesman Lars Hovtun told the AFP news agency. "We still don't know exactly when the orders will come."

Our correspondent says the delay will be a disappointment to the international community.

Co-operation on the chemical weapons removal programme was seen by many of those involved as a potential catalyst for broader peace negotiations in Syria.

Failing to meet this ambitious target, our correspondent adds, will demonstrate the difficulties involved in operating in a country with constantly changing frontlines - even with an international mandate and co-operation from President Assad.

Image caption
The Norwegian cargo ship, Taiko, is due to transport the weapons material

On Saturday, the Joint Mission of the Organisation for the Prohibition of Chemical Weapons (OPCW) and the United Nations acknowledged that while preparations continued, "at this stage, transportation of the most critical chemical material before 31 December is unlikely".

"A number of external factors have impacted upon timelines, not least the continuing volatility in overall security conditions, which have constrained planned movements," a statement said.

The joint mission also noted that the Syrian government had met the 1 November deadline to destroy critical chemical weapons production equipment, which meant it could no longer weaponise the chemical agents at its storage facilities.

On Monday, the US state department stressed that it was "the Assad regime's responsibility to transport the chemicals to the port safely, to facilitate their removal".

But deputy spokeswoman Marie Harf also acknowledged that it was a "complicated process", adding: "As long as we see forward progress, that what's most important here."

In a separate development on Tuesday, activists said a missile fired by government forces hit a bus in Aleppo, killing at least 10 people.

The UK-based Syrian Observatory for Human Rights reported that the dead included two children and that the missile was fired from a plane.

Rebel-held areas of Aleppo have been under intense bombardment since 15 December. More than 500 people have been killed, mainly by barrel-bombs - improvised explosive devices dropped from government aircraft - activists say.